1. Field
The present invention relates to fuel cells, in particular to indirect or redox fuel cells which have applications as power sources for: portable products such as portable electronics products; for transport vehicles such as automobiles, both main and auxiliary; auxiliary power for caravans and other recreational vehicles, boats etc; stationary uses such as uninterruptible power for hospitals computers etc and combined heat and power for homes and businesses. The invention also relates to the use of certain catalysts for use in such fuel cells.
2. Description of the Related Art
Fuel cells have been known for portable applications such as automotive and portable electronics technology for very many years, although it is only in recent years that fuel cells have become of serious practical consideration. In its simplest form, a fuel cell is an electrochemical energy conversion device that converts fuel and oxidant into reaction product(s), producing electricity and heat in the process. In one example of such a cell, hydrogen is used as fuel, and air or oxygen as oxidant and the product of the reaction is water. The gases are fed respectively into catalysing, diffusion-type electrodes separated by a solid or liquid electrolyte which carries electrically charged particles between the two electrodes. In an indirect or redox fuel cell, the oxidant (and/or fuel in some cases) is not reacted directly at the electrode but instead reacts with the reduced form (oxidized form for fuel) of a redox couple to oxidise it, and this oxidised species is fed to the cathode (anode for fuel).
There are several types of fuel cell characterised by their different electrolytes. The liquid electrolyte alkali electrolyte fuel cells have inherent disadvantages in that the electrolyte dissolves CO2 and needs to be replaced periodically. Polymer electrolyte or PEM-type cells with proton-conducting solid cell membranes are acidic and avoid this problem. However, it has proved difficult in practice to attain power outputs from such systems approaching the theoretical maximum level, due to the relatively poor electrocatalysis of the oxygen reduction reaction. In addition expensive noble metal electrocatalysts are often used.
Many current fuel cell technologies employ cathodes where oxygen gas is flowed directly to the electrode where it then reacts with a catalyst to produce water. In many cases the catalyst employed is platinum, a precious metal. Not only does this increase the costs of the overall fuel cell, but the inefficiency of the reaction leads to a loss in available power.
U.S. Pat. No. 3,152,013 discloses a gaseous fuel cell comprising a cation-selective permeable membrane, a gas permeable catalytic electrode and a second electrode, with the membrane being positioned between the electrodes and in electrical contact only with the gas permeable electrode. An aqueous catholyte is provided in contact with the second electrode and the membrane, the catholyte including an oxidant couple therein. Means are provided for supplying a fuel gas to the permeable electrode, and for supplying a gaseous oxidant to the catholyte for oxidising reduced oxidant material. The preferred catholyte and redox couple is HBr/KBr/Br2. Nitrogen oxide is disclosed as a preferred catalyst for oxygen reduction, but with the consequence that pure oxygen was required as oxidant, the use of air as oxidant requiring the venting of noxious nitrogen oxide species.
An acknowledged problem concerning electrochemical fuel cells is that the theoretical potential of a given electrode reaction under defined conditions can be calculated but never completely attained. Imperfections in the system inevitably result in a loss of potential to some level below the theoretical potential attainable from any given reaction. Previous attempts to reduce such imperfections include the selection of catholyte additives which undergo oxidation-reduction reactions in the catholyte solution. For example, U.S. Pat. No. 3,294,588 discloses the use of quinones and dyes in this capacity. Another redox couple which has been tried is the vanadate/vanadyl couple, as disclosed in U.S. Pat. No. 3,279,949.
According to U.S. Pat. No. 3,540,933, certain advantages could be realised in electrochemical fuel cells by using the same electrolyte solution for both catholyte and anolyte. This document discloses the use of a liquid electrolyte containing more than two redox couples therein, with equilibrium potentials not more than 0.8 V apart from any other redox couple in the electrolyte.
The matching of the redox potentials of different redox couples in the electrolyte solution is also considered in U.S. Pat. No. 3,360,401, which concerns the use of an intermediate electron transfer species to increase the rate of flow of electrical energy from a fuel cell. The use of platinum coated electrodes is also disclosed.
Several types of proton exchange membrane fuel cells exist. For example, in U.S. Pat. No. 4,396,687 a fuel cell is disclosed which comprises regenerable anolyte and catholyte solutions. The anolyte solution is one which is reduced from an oxidised state to a reduced state by exposure of the anolyte solution to hydrogen. According to U.S. Pat. No. 4,396,687, preferred anolyte solutions are tungstosilicic acid (H4SiW12O40) or tungstophosphoric acid (H3PW12O40) in the presence of a catalyst.
The preferred catholyte solution of U.S. Pat. No. 4,396,687 is one which is re-oxidised from a reduced state to an oxidized state by direct exposure of the catholyte solution to oxygen. The catholyte of U.S. Pat. No. 4,396,687 includes a mediator component comprising a solution of VOSO4. The mediator functions as an electron sink which is reduced from an oxidation state of V(v) to V(IV). The catholyte also includes a catalyst for regenerating the mediator to its oxidised state, (VO2)2SO4. The catalyst present in the catholyte of U.S. Pat. No. 4,396,687 is a polyoxometallate (POM) solution, namely H5PMo10V2O40.
A significant amount of investigation into the interaction of certain N-donor complexes with oxidants such as hydrogen peroxide or peracids has been reported in the general literature and elsewhere.
WO-A-0012667 describes the use of N-donor complexes as transition metal bleach catalysts which can catalyse the oxidation of stains by air or dioxygen in aqueous solution.
WO0029537 describes the use of transition metal complexes containing cross-bridged macropolycyclic N-donor ligands as bleach catalysts which operate in detergent compositions which are substantially free of any organic or inorganic peroxygen compounds.
A thesis from the University of Groningen entitled ‘Models for non-heme iron containing oxidation enzymes’ by J. G. Roelfes mentions that Fe(N4Py) type complexes are capable of oxygen activation as demonstrated in DNA cleavage experiments.
M. Klopstra, R. Hage, R. M. Kellogg and B. L Fering a, Tet. Lett., 2003, 44, 4581: discusses benzylic oxidation by catalysts such as Fe(N4Py) using O2 as the oxidant. An autoxidation mechanism is proposed where Fe catalyst reacts with 1-phentlethylhydroperoxide.
U.S. Pat. No. 5,298,343 relates to polycomponent electrocatalysts suitable for use at the cathode of electrochemical and fuel cells.
US-A-2005/0112055 discloses a catalyst comprising a di-ruthenium-substituted polyoxometallate.
Prior art fuel cells all suffer from one or more of the following disadvantages:
They are inefficient; they are expensive and/or expensive to assemble; they use expensive and/or environmentally unfriendly materials; they yield inadequate and/or insufficiently maintainable current densities and/or cell potentials; they are too large in their construction; they operate at too high a temperature; they produce unwanted by-products and/or pollutants and/or noxious materials; they have not found practical, commercial utility in portable applications such as automotive and portable electronics.